This is a proposal to conduct a study of theoretically-relevant mediators of cognitive therapy (CT) and supportive-expressive psychodynamic therapy (SE) implemented in community mental health centers. The study will be appended to a recently funded comparative effectiveness study of these two psychotherapies. Theories of the mechanism of change in CT have proposed that CT works by changing (1) dysfunctional attitudes, (2) underlying schemas, and/or (3) compensatory skills. SE therapy theoretically works by changing self-understanding in regard to interpersonal patterns. However, no fully adequate and comprehensive test of these potential mediators has been done to date. In particular, studies of the mechanism of CT have rarely used an alternative psychotherapy as a comparison group. In the parent effectiveness study, patients are randomly assigned to receive 12 weeks of CT or SE therapy, with measures of depressive symptoms, functioning, and quality of life completed monthly. We propose to administer a self-report measure of self-understanding of interpersonal patterns, a self-report measure of dysfunctional attitudes, a rater-scored measure of compensatory skills, and a computerized task that assesses underlying cognitive schemas. These measures will be conducted at baseline, month 1, and month 2 for 210 patients in the parent study. Analyses of mediation will be guided by both Baron & Kenny and MacArthur models, with change in the mediators predicting subsequent change in depressive symptoms and functioning in a longitudinal model, so that results are consistent with a model of change in the mediator causing subsequent improvements on outcome measures. The significance of this work is the potential identification of how both CT and SE therapy work in community settings. Results will have implications for the training of therapists in community settings, potential revisions of the CT and SE models of therapy, and decisions about dissemination of evidence-based treatments to community mental health centers.